PHX787
Topic Author
Posts: 7877
Joined: Thu Mar 15, 2012 7:46 pm

Higher/lower Altitudes And Fuel Burn

Wed Jun 27, 2012 6:43 am

Simple question: when it comes to fuel efficiency, is it preferred to be at a higher altitude (FL37+) or a lower altitude (FL32-) to have better fuel efficiency, or in between?

Does anybody have any stats for ULH planes (747, 777, A340, A380) and their fuel burn in comparison to altitude?


Thanks, Z
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AF1624
Posts: 498
Joined: Wed Jul 12, 2006 6:57 am

RE: Higher/lower Altitudes And Fuel Burn

Wed Jun 27, 2012 7:14 am

It strictly depends on gross weight, CG, temperature and winds.

The FMC (on aircraft equipped with it) will work out an optimum altitude depending on these factors + some others.

As a rule of thumb, higher altitude = thinner air + lower temperature = lower fuel burn

BUT

If the aircraft is too heavy, higher altitude = thinner air = higher angle of attack to keep level flight = higher fuel burn

AND of course winds play a big part in it.

That's pretty much how it works.

Usually, an airliner will do step climbs. i.e. start cruising at FL350, and then as fuel levels diminish weight decreases so 370, then 390, then 410.
Cheers
 
CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: Higher/lower Altitudes And Fuel Burn

Wed Jun 27, 2012 1:11 pm

Our dispatchers also plot say 4 routes and altitudes and choose route/altitude based on the best for time and or fuel burn depending on what's most important for that flight.
 
Mir
Posts: 19093
Joined: Mon Jan 05, 2004 3:55 am

RE: Higher/lower Altitudes And Fuel Burn

Wed Jun 27, 2012 11:53 pm

It is, in general, better to be higher. However, if you're too heavy for a certain altitude, then you'll burn more fuel than you would if you stayed lower down until you became light enough to climb up. Aircraft will have an optimum altitude for a particular weight, and and significant deviation from that will result in higher fuel burn. The optimum altitude will increase over the course of the flight as the aircraft burns off fuel and becomes lighter.

-Mir
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